Using underfill or flux to promote placing and parallel bonding of light emitting diodes

ABSTRACT

Embodiments relate to using flux or underfill as a trapping layer for temporarily attaching light emitting diodes (LEDs) to a substrate and heating to simultaneously bond multiple LEDs onto the substrate. The flux or underfill may be selectively coated at the ends of electrodes of the LEDs prior to placing the LEDs on the substrate. Due to adhesive properties of the flux or underfill, multiple LEDs can be placed on and attached to the substrate prior to performing the bonding process. Once LEDs are placed on the substrate, the flux or underfill facilitates formation of metallic contacts between electrodes of the LED and contacts of the substrate during the bonding process. By using the flux or underfill, the formation of metallic contacts can be performed even without applying pressure.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a division of U.S. application Ser. No. 16/425,866,filed May 29, 2019, which claims the benefit of U.S. ProvisionalApplication No. 62/742,838, filed Oct. 8, 2018. The subject matter ofall of the foregoing is incorporated herein by reference in theirentirety.

BACKGROUND

The present disclosure relates to placing and bonding light emittingdiodes (LEDs) on a substrate, and specifically, to applying a flux orunderfill between the LEDs and substrate to promote the placing andbonding processes.

In display fabrication, LEDs may be moved from one substrate to another.For example, micro-LEDs (μLEDs) for emitting different colors of lightmay be transferred from native substrates (on which the micro-LEDs) arefabricated or carrier substrates to a display substrate includingcontrol circuits for the μLEDs to manufacture an electronic display. Asthe form factor of LEDs decreases, the placing of LEDs into desiredarrangements without damaging the LED dies becomes increasinglydifficult.

Furthermore, LEDs are bonded to a substrate by thermocompression (TC)bonding to form metallic contacts between the LED and substrate. TCbonding forms metallic contacts between two metals by simultaneouslyapplying force and heat. To ensure each LED is bonded correctly, theplacing and bonding process is applied to one LED at a time.Specifically, once an LED is placed on a substrate, it is bonded to thesubstrate before another LED is placed on the substrate. As a result,the substrate and previously bonded LEDs undergo multiple heatingcycles. Repeated high temperature heating cycles is time consuming,increases the risk of damaging LEDs, and can lead to the formation ofoxide layers at the metallic contacts.

SUMMARY

Embodiments relate to using flux or underfill as a trapping layer fortemporarily attaching light emitting diodes (LEDs) to a substrate andheating to simultaneously bond multiple LEDs onto the substrate.Electrodes of a light emitting diode (LED) die are aligned with contactsof a substrate. Flux or underfill is provided on at least the electrodesor the contacts. The LED die is placed on the substrate with the flux orunderfill as a trapping layer between the electrodes and the contacts.The electrodes, the contacts, and the flux or underfill are heated toform a metallic contact between the LED die and the substrate.

In some embodiments, a pick-up head for placing the LED die on thesubstrate is detached from the first LED die after placing the LED dieon the substrate. Adhesive forces of the flux or underfill secure theLED die on the substrate during the detachment of the pick-up head fromthe LED die.

In some embodiments, another LED die is placed on the substrate prior tothe heating process. The other LED die may be placed on the substratesimultaneously with the placing of the LED die on the substrate.

In some embodiments, the metallic contact is formed without applyingexternal pressure on the first LED die towards the substrate during theheating.

In some embodiments, pressure is applied on the first LED die towardsthe substrate by placing the first LED die and the substrate in ahigh-pressure chamber during the heating.

In some embodiments, a platform with an elastomer pad is placed on thefirst LED die. Pressure is applied on the first LED die towards thesubstrate by applying pressure on the platform with the elastomer pad.

In some embodiments, the flux or underfill is provided on theelectrodes, and at least tips of the electrodes are coated with the fluxor underfill by dipping the tips into a flux or underfill layer.

In some embodiments, the flux or underfill is rosin or Benzocyclobutene(BCB).

In some embodiments, subsequent to placing the first LED die on thesubstrate, the first LED die is repositioned to align the electrodeswith the contacts. The flux of underfill remains between the electrodesand the contact.

In some embodiments, the first LED die is repositioned based on imagesignals received from a camera. The camera captures images of the firstLED die through a microscope lens.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1A is a cross sectional view of LED dies bonded to a substrate bymetallic contacts, according to one embodiment.

FIG. 1B is a cross sectional view of LED dies placed on the substrateprior to bonding, according to one embodiment.

FIGS. 2A-2C illustrate a sequence of schematic diagrams for bondingelectrodes of LED dies and contacts of the substrate, according to oneembodiment.

FIGS. 3A-3G illustrate a sequence of schematic diagrams for applyingflux or underfill on electrodes of the LED dies and removing a nativesubstrate from the LED dies, according to one embodiment.

FIG. 4A is a cross sectional view of a platform with elastomer padsattached to LEDs on a substrate, according to one embodiment.

FIG. 4B is a bottom view of the platform with elastomer pads, accordingto one embodiment.

FIG. 5 is a cross sectional view of a high-pressure chamber containingLEDs on a substrate, according to one embodiment.

FIG. 6 is a flow chart illustrating a method for using flux or underfillto temporarily attach and bond an LED to a substrate, according to oneembodiment.

FIG. 7 is a schematic diagram illustrating an operation of a pick-uphead placing an LED onto a substrate, according to one embodiment.

FIG. 8 is a block diagram of the controller, according to oneembodiment.

FIG. 9 is a block diagram of software modules in the memory of thecontroller, according to one embodiment.

FIG. 10 is a circuit diagram of a testing circuit on a substrate,according to one embodiment.

FIGS. 11A through 11C are schematic cross sections of a micro (μLED),according to some embodiments.

The figures depict various embodiments of the present disclosure forpurposes of illustration only.

DETAILED DESCRIPTION

In the following description of embodiments, numerous specific detailsare set forth in order to provide more thorough understanding. However,note that the embodiments may be practiced without one or more of thesespecific details. In other instances, well-known features have not beendescribed in detail to avoid unnecessarily complicating the description.

Embodiments are described herein with reference to the figures wherelike reference numbers indicate identical or functionally similarelements. Also in the figures, the left most digits of each referencenumber correspond to the figure in which the reference number is firstused.

Embodiments relate to using flux or underfill as a trapping layer fortemporarily attaching light emitting diodes (LEDs) to a substrate andheating to simultaneously bond multiple LEDs onto the substrate. Theflux or underfill may be selectively coated at the ends of electrodes ofthe LEDs prior to placing the LEDs on the substrate. Due to adhesiveproperties of the flux or underfill, multiple LEDs can be placed on andattached to the substrate prior to performing the bonding process. OnceLEDs are placed on the substrate, the flux or underfill facilitatesformation of metallic contacts between electrodes of the LED andcontacts of the substrate during the bonding process. By using the fluxor underfill, the formation of metallic contacts can be performedwithout applying pressure.

FIG. 1A is a cross sectional view of LED dies 110 bonded to a displaysubstrate 120 by metallic contacts 130, according to one embodiment.FIG. 1B is a cross sectional view of LED dies 110 placed on the displaysubstrate 120 prior to bonding, according to one embodiment. Theelectrodes 140 of the LED dies 110 are aligned with and temporarilyattached to conducting contacts 150 of the display substrate 120 by theflux or underfill 160. During bonding, part of the electrodes 140, partof the contacts 150, and flux or underfill 160 melt to form the metalliccontacts 130. This can be done in parallel for multiple LEDs 110 placedon the display substrate 120.

In FIG. 1B, the display substrate 120 is on top of a hot plate 170 toheat the electrodes 140, the contacts 150 and the flux or underfill 160.However, different mechanisms may be used to heat the assembly such asexposing the electrodes 140, the contact 150, and the flux or underfill160 to a laser beam.

An LED 110 is a surface-mounted device (SMD) that emits light if avoltage difference is applied between the electrodes 140. The electrodes140 can be made of a single metal (e.g., gold (Au)) or alloys (e.g.,copper (Cu) and tin (Sn) or gold (Au) and tin (Sn)). The electrodes 140may be nanoporous. An LED 110 can have an epitaxial structure formedfrom, among other examples, Gallium nitride (GaN), gallium arsenide(GaAs), or gallium phosphide (GaP). In some embodiments, the LED 110 isa micro-LED (μLED) die. The LED 110 may also be embodied as avertical-cavity surface-emitting laser (VCSEL) that emits infraredwavelengths.

The electrodes 140 of the LED 110 can be aligned with and paced oncontacts of 150 the display substrate 120 by a pick-up head.Alternatively, an array of LEDs 110 and their electrodes 140 can bealigned with and placed on an array of contacts 150 in a single step(this may be referred to as a monolithic approach). In some embodiments,the circuits in the display substrate 120 are powered so that the LED110 emits light 128 as soon as electrical contact is established duringthe placement process, as described with reference to FIGS. 7-10.

The display substrate 120 mechanically supports electronic components(such as the LEDs 110) and electrically connects the electroniccomponents using traces (not shown) and contacts 150. For example, thedisplay substrate is a semiconductor substrate with traces, contacts andother electronic components fabricated using complementarymetal-oxide-semiconductor (CMOS) technology. In some embodiments, thecontacts 150 are alloys that include copper (Cu). The display substrate120 can support any number of LEDs 110. The display substrate 120 mayinclude circuits that are completed once one or more LEDs 110 are placedonto the display substrate 120. In some embodiments, the displaysubstrate 120 can include a control circuit that drives current in thedisplay substrate 120. For example, the display substrate 120 is adisplay substrate of an electronic display. In this example, the LEDsmay be placed (e.g., by a pick-up head) at pixel or sub-pixel locationsto connect the LED dies to control circuits in the display substrate. Inthis way, the control circuit can drive the electronic display byapplying current to the LED dies 110.

The metallic contacts 130 are electrical connections between theelectrodes 140 of the LEDs 110 and the contacts 150 of the displaysubstrate 120. The metallic contacts 130 are formed by bonding theelectrodes 140 to the contacts 150. Depending on the material of theelectrodes and the contacts 150, the metallic contacts 130 may be ametallic bond (e.g., a pure gold or copper bond) or an intermetallicbond (e.g., a gold-tin or copper-tin alloy bond). The metallic contacts130 are formed during the bonding process by heating the electrodes 140,contacts 150, and flux or underfill 160. Pressure may also be applied toform the metallic contacts 130. When both heat and pressure are applied,the process is referred to as thermocompression (TC) bonding. However,applying pressure on LEDs 110 during bonding may cause misalignment.Hence, in some embodiments, the metallic contacts 130 are formed withoutapplying external pressure or applying only reduced pressure to the LEDs110 towards the display substrate 120 during the bonding process.

The hot plate 170 is a plate that can control the temperature of theelectrodes 140, contacts 150, and flux or underfill 160 by heating orcooling the display substrate 120. The hot plate 170 may be advantageousfor bonding the contacts 150 to the electrodes 140. For example, due toheat from the hot plate 170, the electrodes 140, contacts 150, and fluxor underfill 160 melt to form the metallic contact 130. The hot plate170 may be a Peltier cell when bonding temperatures are low (e.g., −20°C. to 90° C.). The bonding temperatures may be low when the flux orunderfill includes rosin. In some embodiments, the temperature of theelectrodes 140, contacts 150, and flux or underfill 160 is controlled byanother (or additional) method or apparatus, such as a laser beam thatlocally heats the electrodes 140, contacts 150, and flux or underfill160. For example, after multiple LEDs 110 are placed, a laser setup maybe employed to selectively heat individual LEDs 110. A laser setup maybe employed when bonding temperatures are high (e.g., 250° C. to 300°C.). Bonding temperatures may be high to bond metals (e.g., to form acopper-tin bond).

The flux or underfill 160 promotes bonding of the electrodes 140 to thecontacts 150 to form the metallic contacts 130. The flux or underfill160 placed between the electrodes 140 and contacts 150 may be referredto herein as a ‘trapping layer’ because the flux or underfill 150temporarily holds the electrodes 140 in place so that the electrodes 140can bond with the contacts 150. The flux or underfill 160 is anycombination of underfill material, flux material, and underfill materialwith flux properties. Examples of flux or underfill 160 include, but arenot limited to, rosin or other similar flux types, epoxy basedmaterials, and Benzocyclobutene (BCB) based materials. Examples of othermaterials that can be used as a trapping layer include, but are notlimited to, conductive paste formulations (e.g., silver nanoparticleink), low melting point metals (e.g., indium), nanoporous gold, andeutectic alloys. Flux material remove oxides (e.g., Cu or Sn oxides)during the bonding process because the oxides may prevent formation themetallic contacts 130. Thus, in embodiments where oxidation does notoccur or is reduced during the bonding process (e.g., the electrodes 140and contacts 150 are gold or the bonding process occurs in a reducedatmosphere environment), the flux or underfill 160 may not include fluxmaterial. In some embodiments, the flux or underfill 160 mechanicallystrengthens the bond structure. During the bonding process, the flux orunderfill 160 may turn liquid around 50° C., may become active byremoving oxides around 80-110° C., and may assist bonding by decreasingsurface tension around 230° C.

In some embodiments, if flux or underfill 160 is placed between theelectrodes 140 and contacts 150, the bonding process can form metalliccontacts 130 without applying external pressure on the LEDs 110 towardsthe display substrate 120. During heating, the flux or underfill 160 candecrease the surface tension of the melted electrodes 140 and contacts150. The flux or underfill 160 can pull the opposing surfaces of theelectrodes 140 and contacts 150 together due to capillary forces andmass loss during thermal decomposition. This may particularly occur whenthe dimensions of the LED dies 110 are on the order of micrometers. Theflux or underfill 160 can also allow effective wetting of both surfacesof the electrodes 140 and contacts 150. Adhesive properties of the fluxor underfill 160 can maintain the position of the placed LED 110 on thesubstrate (e.g., the flux or underfill 160 maintains contact between theelectrodes 140 and contacts 150) during the bonding process. Due to anycombination of these features, the metallic contacts 130 can be formedwithout applying external pressure to the LEDs 110.

In some embodiments, the flux or underfill functions as the trappinglayer having adhesive properties to assist in the temporary placement ofthe LEDs 110 on the display substrate 120. For example, flux orunderfills 160 can be become solid below certain temperatures (e.g.,rosin is solid below 50° C.), allowing multiple LEDs 110 to be placed onthe display substrate 120 prior to the bonding process. For example,after placement of an LED 110 by a pick-up head, the pick-up head candetach from the LED 110 due the adhesive forces of the flux or underfill160 keeping the LED 110 attached to the display substrate 120, so thatany number of LEDs 110 can form metallic contacts 130 with the displaysubstrate 120 during a single bonding process. Among other advantages, asingle bonding process for forming metallic contacts 130 can reduce aneed to remove oxide layers formed on metallic contacts 130 aftermultiple thermal cycles. A single bonding process can also reduce a riskof damaging the LEDs 110 from multiple thermal cycles. A single bondingprocess can also be less time consuming than multiple bonding processes.A single bonding process can also reduce a risk of dendrites formation.

Furthermore, the temporary attachment of LEDs 110 to the contacts 150 bythe flux or underfill 160 can allow LEDs 110 to be repositioned afteralignment. For example, after multiple LEDs 110 are aligned and placed,misplaced LEDs 110 can be repositioned prior to bonding.

FIGS. 2A-2C illustrate a sequence of schematic diagrams for bondingelectrodes 140 of LED dies 110 and contacts 150 of the display substrate120, according to one embodiment. FIG. 2A is a cross sectional view offlux or underfill 160 applied to the electrodes 140 prior to placementof the LEDs 110 on the display substrate 120, according to oneembodiment. FIG. 2B is a cross sectional view of flux or underfill 160applied to the contacts 150 prior to placement of the LEDs 110 on thedisplay substrate 120, according to one embodiment. Among otheradvantages, application of the flux or underfill 160 on the electrodes140 or the contacts 150 can reduce cleaning of the flux or underfill 160from the display substrate 120 after the bonding process. FIG. 2C is across sectional view of flux or underfill 160 coated on the displaysubstrate 120 prior to placement of the LEDs 110 on the displaysubstrate 120, according to one embodiment. The flux or underfill can beapplied to the display substrate 120 by spin coating. After the LEDs 110are bonded to the display substrate 120, remaining flux or underfill maybe removed, such as by etching or application of a solvent.

FIGS. 3A-3G illustrate a sequence of schematic diagrams for applyingflux or underfill 160 on electrodes 140 of the LED dies 110 (e.g., asseen in FIG. 2A) and removing a native substrate 310 from the LED dies110, according to one embodiment. FIG. 3A is a cross sectional view of aflux or underfill layer 330 on a slide 320 and LEDs 110 attached to anative substrate 310, according to one embodiment. The native substrate310 can be the substrate that the LEDs 110 were formed on. For example,the native substrate is a sapphire substrate (e.g., GaAs). The flux orunderfill layer 330 can be deposited on the slide 320 by spin coating.For example, the slide 320 is made of glass slide coated with rosin asthe flux or underfill layer 330.

FIG. 3B is a cross sectional view of electrodes 140 of the LEDs 110dipped into the flux or underfill layer 330, according to oneembodiment. FIG. 3C is a cross sectional view of LEDs 110 removed fromthe flux or underfill layer 330, according to one embodiment. As resultof dipping the electrodes 140 of the LEDs 110 into the flux or underfilllayer 330, the flux or underfill 160 is applied to at least tips of theelectrodes 140.

FIG. 3D is a cross sectional view of LEDs 110 attached to the nativesubstrate 310 embedded in a polymer 340 on a carrier substrate 350,according to one embodiment. FIG. 3E is a cross sectional view of LEDs110 embedded in a polymer 340 on the carrier substrate 350 and detachedfrom the native substrate 310, according to one embodiment. The nativesubstrate 310 can be removed by wet etching or laser lift off (LLO), forexample. After the native substrate 310 is removed, the carriersubstrate 350 with the LEDs 110 can be moved to a different facility orlocation for further processing of the LEDs 110. During such movingprocess, the polymer 340 firmly holds the LEDs 110 in place.

FIG. 3F is a cross sectional view of LEDs 110 attached to an etchedpolymer 360, according to one embodiment. After the native substrate 310is removed and the carrier substrate 350 is moved to a desiredprocessing facility or location, portions of the polymer 340 areremoved. Portions can be removed by etching, such as radio frequency(RF) dry etching, to create the etched polymer 360. Alternatively,portions of the polymer 340 can be removed by application of a solventthat dissolves the polymer 340.

After removing portions of the polymer 340, the LEDs 110 can be detachedfrom the etched polymer 360, as illustrated in FIG. 3G. By removingportions of the polymer 340, the polymer 340 no longer holds the LEDs110 firmly in place, and enables the LEDs 110 to be detached from thecarrier substrate 340 (e.g., by a pick-up head 370). After detachingfrom the etched polymer 360, the LEDs 110 with the flux or underfill 160can be aligned and placed on a display substrate 120. A pick-up head 370and aligning and placing LEDs 110 on a display substrate 120 are furtherdescribed with reference to FIGS. 7-10.

Due to adhesive properties of the flux or underfill 160, the flux orunderfill 160 allows multiple LEDs (e.g., all desired LEDs) to be placedprior to bonding. After an LED 110 is placed, adhesive properties of theflux or underfill 160 can temporarily hold the LED 110 in place,attached to the display substrate 120. The adhesive properties of theflux or underfill 150 allow a pick-up head 370 to detach from an LED 110placed on the display substrate 120 and perform subsequent pick andplace operations multiple times. After the LEDs 110 are aligned andplaced on the display substrate 120, the LEDs 110 can be simultaneouslybonded in parallel to the display substrate 120 by forming metalliccontacts 130. In some embodiments, pressure is applied to the LEDs 110and display substrate 120 during the bonding process to assist informing metallic contacts 130, as described below in detail withreference to FIGS. 4A-5.

FIG. 4A is a cross sectional view of a platform 410 with elastomer pads420 attached to LEDs 110 on a display substrate 120, according to oneembodiment. FIG. 4B is a bottom view of the platform 410 with elastomerpads 420, according to one embodiment. The platform 410 with elastomerpads 420 can be used to apply pressure (e.g., uniformly) on the LEDs 110towards the display substrate 120 by applying pressure on the platform410. Alternative pick and place methods can be used other than aplatform with elastomer pads, such as mechanical gripers or vacuumchucks.

In some embodiments, each elastomer pad 420 is in contact with a singleLED 110. Due to the discrete elastomer pads 420, lateral movement of theLEDs 110 on the display substrate 120 can be proportional to thecoefficient of thermal expansion (CTE) of the platform 410. Thus, if theplatform has a negligible CTE, such as fused silica platform, thelateral movement of the LEDs 110 during the bonding process can bereduced.

FIG. 5 is a cross sectional view of a high-pressure chamber 510containing LEDs 110 on a display substrate 120, according to oneembodiment. By increasing the internal pressure of the high-pressurechamber 510, the ambient pressure on the LEDs 110 and display substrate120 increases. Due to Pascal's law, the increase in ambient pressureresults in a net downward force on the LEDs 110, due to a difference insurface area between the bottom and top surfaces of the LEDs 110.

During the bonding process, a hot plate 170 or heating system (neithershown in FIG. 5) can heat the gas within the high-pressure chamber 150.For example, a heating system increases the gas in the chamber to 300°C. Among other advantages, since no solid object applies pressure on theLEDs 110, the high-pressure chamber can reduce lateral movement of theLEDs 110 during the bonding process. For example, lateral movement canbe caused by a CTE mismatch when a solid object applies pressure on theLEDs 110. Thus, lateral alignment of the placed LEDs 110 on the displaysubstrate 120 can be preserved when forming the metallic contacts 130.

FIG. 6 is a flow chart illustrating a method for using flux or underfillto temporarily attach and bond an LED to a substrate, according to oneembodiment. The steps of method may be performed in different orders,and the method may include different, additional, or fewer steps.

Electrodes of a first light emitting diode (LED) die are aligned 610with contacts of a substrate. Flux or underfill is provided on at leastthe electrodes or the contacts. In some embodiments, the flux orunderfill is rosin.

The first LED die is placed 620 on the substrate. The flux or underfillforms a trapping layer between the electrodes and the contacts. In someembodiments, the LED die is aligned with and placed on the substrate bya pick-up head. After placing the LED die on the substrate, the pick-uphead is detached from the LED die due to adhesive forces of the flux orunderfill securing the first LED die to the substrate. For example, thepick-up head is performing a pick and place operation for multiple LEDdies.

The electrodes, contacts, and flux or underfill are heated 630 to form ametallic contact between the first LED die and the substrate. In someembodiments, the electrodes, the contacts, and the flux or underfill areselectively heated by focusing laser light.

In some embodiments, the flux or underfill is provided on theelectrodes, and at least tips of the electrodes are coated with the fluxor underfill by dipping the tips into a flux or underfill layer. In someembodiments, the electrodes having at least the tips coated with theflux or underfill are embedded in a polymer. After embedding theelectrodes in a polymer, a native substrate is removed from the firstLED die. The native substrate is a substrate on which the first LED diewas fabricated. The native substrate is removed prior to aligning theelectrodes with the contacts. In some embodiments, portions of thepolymer surrounding the electrodes are etched. After etching theportions of the polymer, the first LED die is picked up by a pick-uphead for aligning and placing the first LED die on the substrate.

FIG. 7 is a cross sectional view illustrating an operation of a pick-uphead 370 placing an LED 110 onto a display substrate 120, according toone embodiment. The pick-up head 370 is attached to the LED 110 andplaces the LED 110 onto the display substrate 120 by aligning contacts150 of the display substrate 120 with electrodes 140 of the LED 110. Ifa voltage difference is applied between the contacts 150 and if the LED110 is properly placed, the LED 110 can emit light 728 from an emissionsurface 730. A hot plate 170 is connected to the display substrate 120.A camera 712 is placed above a microscope lens 710 to capture images ofthe LED 110 being placed onto the display substrate 120 from the top.The camera 712 generates image signals 714 that function as real timefeedback to correct improper LED 110 placement during the placementprocess. The camera 712 sends image signals 714 to the controller 716.Using the image signals 714, the controller 716 sends control signals718 to the actuator 720. The actuator is attached to the pick-up head370 via a mount 708. In some embodiments, FIG. 7 includes differentand/or other components than those shown in FIG. 7. For example, the LED110 can include an elastomeric material layer that allows the LED 110 tobe adhesively attached to a pick-up surface of the pick-up head 370. Inanother example, the LED 110 can be temporarily attached to the pick-uphead by mechanical gripers or vacuum chucks.

The pick-up head 370 places LEDs 110 onto the display substrate 120. Thepick-up head 370 may also be referred to as a pick and place head. Thepick-up head 370 can support any number of LEDs 110 and can placemultiple LEDs 110 onto the display substrate 120 at once. For example,an array of LEDs 110 and their electrodes 140 can be aligned with andplaced on an array of contacts 150 in a single step (this may bereferred to as a monolithic approach). Before placing the LED 110, thepick-up head 370 may pick up the LED 110 from a native substrate or acarrier substrate 350. Picking up an LED 110 from a native or carriersubstrate 350 and aligning and placing the LED 110 on the displaysubstrate 120 can be referred to as a pick and place operation. Due tothe flux or underfill 160, the pick-up head 370 can perform multiplepick and place operations without bonding each LED 110 to the displaysubstrate 120. In some embodiments, a portion of the pick-up head 370 istransparent to allow the camera 712 to capture images of the LED 110through the pick-up head 370. In some embodiments, one or more LEDs 110are repositioned once they are positioned on the display substrate 120,for example, because the electrodes 140 are misaligned with the contacts150. In these embodiments, the flux or underfill can be flexible enoughto stay between the electrodes 140 and the contacts 150.

The mount 708 is an actuated slide that supports the pick-up head 370.The mount 708 can support multiple pick-up heads 706. For example, themount 708 supports two pick-up heads 706 such that two LEDs 110 can beplaced at once. In some embodiments, the mount 708 is made of atransparent material, such as glass.

The actuator 720 is connected to the mount 708 and controls movement ofthe mount 708. By moving the mount 708, the actuator 720 aligns thepick-up head 370 with the display substrate 120. This allows the pick-uphead 370 to place one or more LEDs 110 on the display substrate 120 byaligning the electrodes 140 with the contacts 150. In some embodiments,the actuator 720 is a multiple degree of freedom actuator, such as anactuator that is configured to move the mount 708 up and down, left andright, forward and back. The actuator can also adjust yaw, tilt, orrotate the mount 708. In some embodiments, multiple actuators 720connected to multiple mounts 708 perform LED 110 placement tasks inparallel to increase throughput.

The camera 712 is an image capturing device that captures the images ofthe LED 110. In some embodiments, the camera 712 captures images todetermine whether the LED 110 is emitting light 728. The camera 712 canalso capture images to determine the placement location and angle of aplaced LED 110. The camera 712 can also enable detection of luminance ofthe light 728 emitted by the LED 110.

The microscope lens 710 magnifies the LED 110 for the camera 712. Themicroscope lens 710 can allow the camera 712 to view and distinguishlight 728 from LEDs 110.

The controller 716 is a computing device that controls the placement ofLEDs 110 by providing control signals 718 to the actuator 720. Thecontrol signals 718 are determined by the controller 716 and can bebased on the image signals 714 received from the camera 712. Thecontroller 716 can analyze the emitting state or the placement of theLED 110 to determine if the placement of the LED 110 should be adjusted.The controller 716 is further described with reference to FIGS. 8 and 9.

FIG. 8 is a block diagram of the controller 716, according to oneembodiment. The controller 716 may include, among other components, aprocessor 802, a memory 804, a user interface 806, a video interface870, and a control interface 808. The modules 802 through 808communicate via a bus 874. Some embodiments of the controller 716 havedifferent and/or other components than those shown in FIG. 8.

The controller 716 is a computer device that may be a personal computer(PC), a video game console, a tablet PC, a smartphone, or any machinecapable of executing instructions (sequential or otherwise) that specifyactions to be taken by that device. The controller 716 can operate as astandalone device or a connected (e.g., networked) device that connectsto other machines. Furthermore, while only a single device isillustrated, the term “device” shall also be taken to include anycollection of devices that individually or jointly execute instructionsto perform any one or more of the methodologies discussed herein.

The processor 802 is a processing circuitry configured to carry outinstructions stored in the memory 804. For example, the processor 802can be a central processing unit (CPU) and/or a graphics processing unit(GPU). The processor 802 may be a general-purpose or embedded processorusing any of a variety of instruction set architectures (ISAs). Althougha single processor 802 is illustrated in FIG. 8, the controller 716 mayinclude multiple processors 802. In multiprocessor systems, each of theprocessors 802 may commonly, but not necessarily, implement the sameISA. The processor 802, or a part of it, may be specifically designedfor efficient processing of graphical images, such as those received inthe image signals 714. For example, the processor 802 may perform one ormore image processing steps to determine an emitting state of an LED110.

The memory 804 is a non-transitory machine-readable medium on which isstored data and instructions (e.g., software) embodying any one or moreof the methodologies or functions described herein. For example, thememory 804 may store instructions that when executed by the processor802 configures the processor 802 to perform the method described belowin detail with reference to FIG. 6. Instructions may also reside,completely or at least partially, within the processor 802 (e.g., withinthe processor's cache memory) during execution thereof by the controller716.

The term “machine-readable medium” should be taken to include a singlemedium or multiple media (e.g., a centralized or distributed database,or associated caches and servers) able to store instructions. The term“machine-readable medium” shall also be taken to include any medium thatis capable of storing instructions for execution by the device and thatcause the device to perform any one or more of the methodologiesdisclosed herein. The term “machine-readable medium” includes, but isnot limited to, data repositories in the form of solid-state memories,optical media, and magnetic media.

The user interface 806 is hardware, software, firmware, or a combinationthereof that enables a user to interact with the controller 716. Theuser interface 806 can include an alphanumeric input device (e.g., akeyboard) and a cursor control device (e.g., a mouse, a trackball, ajoystick, a motion sensor, or other pointing instrument). For example, auser uses a keyboard and mouse to select placement parameters forplacing a set of LEDs 110 on the display substrate 120.

The control interface 808 transmits control signals 718 to the actuator720. For example, the control interface 808 is a circuit or acombination of circuits and software that interfaces with the actuator720 to transmit the control signals 718.

The video interface 870 is a circuit or a combination of circuit andsoftware that receives image data via the image signals 714 from thecamera 712 and transfers the image data to the memory 804 and/orprocessor 802 to be stored and processed.

The controller 716 executes computer program modules for providingfunctionality described herein. As used herein, the term “module” refersto computer program instructions and/or other logic used to provide thespecified functionality. Thus, a module can be implemented in hardware,firmware, and/or software. In some embodiments, program modules formedof executable computer program instructions are loaded into the memory804, and executed by the processor 802. For example, programinstructions for the method 700 describe herein can be loaded into thememory 804, and executed by the processor 802.

FIG. 9 is a block diagram of software modules in the memory 804 of thecontroller 716, according to one embodiment. The memory 804 may store,among other modules, an actuator control module 902, a temperaturecontrol module 904, a vision recognition module 906, and a parameteradjuster module 908. The memory 804 may include other modules notillustrated in FIG. 9.

The actuator control module 902 provides instructions for generatingcontrol signals 718 to control the actuator 720 to perform pick andplace operations and adjust one or more placement parameters. Theplacement parameters are parameters that relate to placing one or moreLEDs 110 on the display substrate 120. The placement parameters includea placing location, a placing angle (e.g., including a rotation angleand three tilt angles), a placing pressure, a placing temperature, and aplacing time. The placing location is the location of the LED 110 on thedisplay substrate 120. The placing angle is the angle of the LED 110relative to the display substrate 120. The placing pressure is thepressure applied to the LED 110 by the pick-up head 370 once it isplaced on the display substrate 120. The placing time is the amount oftime that the placing pressure and the placing temperature are appliedto the LED 110. The placing temperature is the temperature of thedisplay substrate 120 or a temperature change of the display substrate120 during the placing of the LED 110.

The temperature control module 904 sets the temperature of the hot plate170. As such, the temperature control module 904 sets the placingtemperature. The temperature control module 904 can also set thetemperature of the hot plate 170 during the bonding process.

Parameters that relate to placement and bonding include heating rampprofile, flux or underfill behavior, underfill behavior, the influenceof lateral and vertical movements (e.g. caused by thermal expansion),the influence of metal oxides, allowable pressure range, and allowabletemperature range.

The heating ramp profile represents the temperature evolution duringbonding. For example, the temperature can increase at a rate of 3°Celsius per second (C/s) up to 750° C., then increase at a rate of 70°C./s up to 270° C., then remain constant for five minutes (so calleddwell time), then decrease at a controlled rate of 2° C./s. The heatingramp profile can be optimized experimentally and/or based on theoreticalsimulations.

Since underfills and fluxes can be liquid and freely move (e.g., whenheated), their presence and evolution during bonding can be opticallymonitored during the bonding process (e.g., using the same opticalfeedback system for LED alignment and placement).

Lateral and vertical movement can occur as a result of heat expansionduring the bonding process. For example, the display substrate 120 andhot plate 170 can expand as their respective temperatures increase. Theamount of expansion can depend on the coefficient of thermal expansion(CTE) of each material (e.g. it is proportional to the temperature andoccurs in all directions).

Lateral and vertical movements may be monitored and controlled duringthe bonding process. For example, when applying pressure by a platform410 with elastomer pads 420, vertical movements may be monitored suchthat the pressure between the display substrate 120 and the LED 110remains constant during bonding.

The vision recognition module 906 performs analysis on the image data inthe image signals 714 to determine the emitting states of the LED 110and the placement of the LED 110. The vision recognition module 906 candetermine whether an emitting state fails one or more criteria. Thecriteria can form a standard for determining proper placement of one ormore LEDs 110. For example, one of the criteria relates to whether theLED 110 emits light 728 or the LED 110 emits an amount of lumens above athreshold. An emitting state can fail the criteria for any number ofreasons, such as, for example, an LED 110 is placed outside a targetplacing location, placing angle, placing time, placing pressure, placingtemperature, etc.

The parameter adjuster module 908 provides instructions for monitoringthe placement and bonding parameters and adjusting them in real time asneeded. For example, the parameter adjuster module 908 adjusts theplacement parameters in response to one or more emitting states failingcriteria. The parameter adjuster module 908 may determine whichparameters to adjust based on the failed criteria. For example, if anLED 110 is incorrectly placed on the display substrate 120 (e.g.,between contacts 150), the placing location can be adjusted. In anotherexample, if the LED 110 moves after placement, the placing time andpressure may be adjusted. The adjusted parameters can be temporarilyadjusted for the LED 110 currently being placed or permanently adjustedfor the current and future placement of LEDs 110. The parameter adjustermodule 908 can continually adjust the placement parameters until one ormore emitting states satisfy the criteria. This can allow for optimizingthe placement parameters of the placement process and allow insight intoroot causes of failed LED 110 placement.

FIG. 10 is a circuit diagram of a testing circuit 1000 on a displaysubstrate 120, according to one embodiment. The testing circuit 1000 canbe used for experimental purposes, for example, to test a new placingscheme or new placement parameters. The arrangement of the testingcircuit 1000 allows parallel testing of the LEDs 110 on the circuitusing only two connecting wires. The testing circuit 1000 includes a topwire 1004, a bottom wire 1006, LEDs 110 a through 704 e (represented asdiodes), and resistors 1002 electrically connected between the top wire1004 and LEDs 110 a through 704 e. The dashed lines around LEDs 110 band 704 d represent shorts between the contacts 150 on the displaysubstrate 120.

By applying a high supply voltage V_(a) to the top wire 1004 and lowsupply voltage V_(b) to the bottom wire 1006 (V_(a)>V_(b)), a voltagebias can be applied across the LEDs 110. As a result, LEDs 110 a, 704 c,and 704 e will emit light 728. However, LEDs 110 b and 704 d will notemit light 728, due to the shorts. Despite the local circuit shorts nearLED 110 b and LED 110 d, the resistors 1002 prevent the entire testingcircuit 1000 from shorting. For example, the resistors 1002 each have aresistance of 750 KΩ.

Furthermore, the relative voltage levels of applied voltages V_(a),V_(b) can be reversed to apply a negative voltage bias across the topand bottom wires (V_(a)<V_(b)). By doing so, reverse current I_(Rev)flows from the bottom wire 1006 to the upper wire 1004 via the shortedLEDs 110 b and 704 d. The number of shorted LEDs in the testing circuit1000 can be estimated by measuring I_(Rev). For example, if each shortedLED allows 70 μA of current to pass through, 20 μA of reverse currentI_(Rev) may indicate that two LEDs are shorted.

Assuming that the LEDs 110 are functioning properly (e.g., they weretested before being picked up by the pick-up head 370), the results ofthe positive and negative voltage bias can be used in combination withthe images captured by the camera 712 to determine the number ofimproperly placed LEDs 110. For example, if no I_(Rev) is measured, yetone or more LEDs 110 do not emit light 728 when current is in theforward direction (V_(a)>V_(b)), then it may be determined that one ormore LEDs 110 were improperly placed on the display substrate 120.

FIGS. 11A through 11C show schematic cross sections of a μLED 1100,according to some embodiments. The μLED 1100 is an example of a visibleor non-visible LED that may be positioned on a surface of a displaysubstrate (e.g., display substrate 120) to emit collimated visible orinvisible light. The feature size of the μLED 1100 (e.g., the diameter)may range from sub-micrometers to tens of micrometers (e.g., from 0.1 μmto 10 μm). The pitch (e.g., spacing between μLEDs) may similarly rangefrom sub-micrometers to tens of micrometers.

The μLED 1100 may be formed on a substrate layer 1102, and may include,among other components, a gallium semiconductor layer 1104 disposed onthe substrate layer 1102, a dielectric layer 1114 disposed on thegallium semiconductor layer 1104, a p-contact 1116 disposed on a firstportion of the dielectric layer 1114, and an n-contact 1118 disposed ona second portion of the gallium semiconductor layer 1104. In someembodiments, the gallium semiconductor layer 1104 is grown on thesubstrate layer 1102 as an epitaxial layer.

As illustrated in FIG. 11B, the substrate layer 1102 may be removed fromthe surface of the gallium semiconductor layer 1104 of the μLED 1100 toreveal a light emitting face 1110 of the μLED 1100. In some embodiments,the substrate layer 1102 is separated from the gallium semiconductorlayer 1104 using a laser lift-off (LLO) process.

In some embodiments, the gallium semiconductor layer 1104 is shaped intoa mesa 1106. An active (or light emitting) layer 1108 (or “active lightemitting area”) is included in the structure of the mesa 1106. The mesa1106 has a truncated top, on a side opposed to the light transmitting oremitting face 1110 of the μLED 1100. The mesa 1106 also has anear-parabolic shape to form a reflective enclosure for light generatedwithin the μLED 1100.

FIG. 11C illustrates the μLED 1100 after removal of the substrate layer1102. Upon removal of the substrate layer 1102, the μLED 1100 may beplaced on a display substrate (not shown), and operated to emit light.The arrows 1112 show how light emitted from the active layer 1108 isreflected off the p-contact 1116 and internal walls of the mesa 1106toward the light emitting face 1110 at an angle sufficient for the lightto escape the μLED device 1100 (i.e., within an angle of total internalreflection). During operation, the p-contact 1116 and the n-contact 1118connect the μLED 1100 to a display substrate (not shown).

In some embodiments, the parabolic shaped structure of the μLED 1100results in an increase in the extraction efficiency of the μLED 1100into low illumination angles when compared to unshaped or standard LEDs.For example, standard LED dies generally provide an emission full widthhalf maximum (FWHM) angle of 120°, which is dictated by the Lambertianreflectance from a diffuse surface. In comparison, the μLED 1100 can bedesigned to provide controlled emission angle FWHM of less than standardLED dies, such as around 60°. This increased efficiency and collimatedoutput of the μLED 1100 can produce light visible to the human eye withonly nano-amps of drive current.

The μLED 1100 may include an active light emitting area that is lessthan standard LEDs, such as less than 2,000 μm². The μLED 1100directionalizes the light output from the active light emitting area andincreases the brightness level of the light output. The μLED 1100 may beless than 20 μm in diameter with a parabolic structure (or a similarstructure) etched directly onto the LED die during the wafer processingsteps to form a quasi-collimated light beam emerging from the lightemitting face 1110 of the μLED 1100.

As used herein, “directionalized light” includes collimated andquasi-collimated light. For example, directionalized light may be lightthat is emitted from a light generating region of a LED and at least aportion of the emitted light is directed into a beam having a halfangle. This may increase the brightness of the LED in the direction ofthe beam of light.

A μLED 1100 may include a circular cross section when cut along ahorizontal plane as shown in FIGS. 11A-11C. A μLED 1100 may have aparabolic structure etched directly onto the LED die during the waferprocessing steps. The parabolic structure may comprise a light emittingregion of the μLED 1100 and reflects a portion of the generated light toform the quasi-collimated light beam emitted from the light emittingface 1110.

As discussed above, the substrate layer 1102 may correspond to a glassor sapphire substrate. The gallium semiconductor layer 1104 may includea p-doped GaN layer, an n-doped GaN layer, and the active layer 1108between the p-doped and n-doped GaN layers. The active layer may includea multi-quantum well structure. The substrate layer 1102 is transparentto a laser projected by the laser projector 126, which may be appliedthrough the substrate layer 1102 to the gallium semiconductor layer1104. In other embodiments, the substrate layer 1102 may comprise agallium compound, as such GaAs. The gallium semiconductor layer 1104 mayinclude a p-doped GaAn layer, an n-doped GaAs layer, and the activelayer 1108 between the p-doped and n-doped GaAs layers. In someembodiments, the μLED 1100 includes a Gallium phosphide (GaP) substrate1102 for increased transparency relative to GaAs, such as for redvisible LEDs. In some embodiments, the substrate layer 1102 is asemiconductor substrate, such as a silicon substrate. When anon-transparent substrate layer 1102 is used, a laser may be applied atthe interface of the substrate layer 1102 and the gallium semiconductorlayer to separate the layers and form the gallium material to facilitatepick and place.

While particular embodiments and applications have been illustrated anddescribed, it is to be understood that the invention is not limited tothe precise construction and components disclosed herein and thatvarious modifications, changes and variations which will be apparent tothose skilled in the art may be made in the arrangement, operation anddetails of the method and apparatus disclosed herein without departingfrom the spirit and scope of the present disclosure.

1. A light emitting assembly comprising: a substrate with contacts on aside of the substrate; and a plurality of light emitting diodes (LEDs)having electrodes connected to the contacts of the substrate by metalliccontacts, a metallic contact for a first light emitting diode (LED) dieformed by: placing at least the first LED die on the substrate with aflux or underfill as a trapping layer between an electrode of the firstLED die and a contact of the substrate; and heating the contact, theelectrode, and the flux or underfill to form the metallic contactbetween the first LED die and the substrate.
 2. The light emittingassembly of claim 1, wherein forming the metallic contact furthercomprises detaching a pick-up head for placing the first LED die on thesubstrate from the first LED die after placing the first LED die on thesubstrate, adhesive forces of the flux or underfill securing the firstLED die on the substrate during detaching of the pick-up head from thefirst LED die.
 3. The light emitting assembly of claim 1, whereinforming the metallic contact further comprises placing a second LED dieon the substrate prior to the heating.
 4. The light emitting assembly ofclaim 1, wherein the metallic contact is formed without applyingexternal pressure on the first LED die towards the substrate during theheating.
 5. The light emitting assembly of claim 1, wherein forming themetallic contact further comprises: placing a platform with an elastomerpad on the first LED die; and applying pressure on the first LED dietowards the substrate by applying pressure on the platform with theelastomer pad.
 6. The light emitting assembly of claim 1, wherein theelectrode, the contact, and the flux or underfill are selectively heatedby focusing laser light.
 7. The light emitting assembly of claim 1,wherein the flux or underfill is rosin.
 8. (canceled)
 9. The lightemitting assembly of claim 3, wherein the second LED die is placed onthe substrate simultaneously with the placing of the first LED die onthe substrate.
 10. The light emitting assembly of claim 1, wherein theflux or underfill is provided on the electrode, and at least a tip ofthe electrode is coated with the flux or underfill by dipping the tipinto a flux or underfill layer.
 11. The light emitting assembly of claim1, wherein forming the metallic contact further comprises subsequent toplacing the first LED die on the substrate, repositioning the first LEDdie to align the electrode with the contact, the flux or underfillremaining between the electrode and the contact.
 12. A non-transitorycomputer readable storage medium with instructions that, when executedby at least one processor, cause the processor to: align electrodes of afirst light emitting diode (LED) die with contacts of a substrate, fluxor underfill provided on at least the electrodes or the contacts; placethe first LED die on the substrate with the flux or underfill as atrapping layer between the electrodes and the contacts; and heat theelectrodes, the contacts, and the flux or underfill to form metalliccontacts between the first LED die and the contacts.
 13. Thenon-transitory computer readable storage medium of claim 12, furthercomprising an instruction to detach a pick-up head for placing the firstLED die on the substrate from the first LED die after placing the firstLED die on the substrate, adhesive forces of the flux or underfillsecuring the first LED die on the substrate during detaching of thepick-up head from the first LED die.
 14. The non-transitory computerreadable storage medium of claim 12, further comprising an instructionto place a second LED die on the substrate prior to the heating.
 15. Thenon-transitory computer readable storage medium of claim 14, wherein thesecond LED die is placed on the substrate simultaneously with theplacing of the first LED die on the substrate.
 16. The non-transitorycomputer readable storage medium of claim 12, wherein the flux orunderfill is provided on the electrodes, and at least tips of theelectrodes are coated with the flux or underfill by dipping the tipsinto a flux or underfill layer.
 17. The non-transitory computer readablestorage medium of claim 12, wherein the metallic contacts are formedwithout applying external pressure on the first LED die towards thesubstrate during the heating.
 18. The non-transitory computer readablestorage medium of claim 12, further comprising instructions to: place aplatform with an elastomer pad on the first LED die; and apply pressureon the first LED die towards the substrate by applying pressure on theplatform with the elastomer pad.
 19. The non-transitory computerreadable storage medium of claim 12, wherein the electrodes, thecontacts, and the flux or underfill are selectively heated by focusinglaser light.
 20. The non-transitory computer readable storage medium ofclaim 12, wherein the flux or underfill is rosin.
 21. The non-transitorycomputer readable storage medium of claim 12, further comprising aninstruction to, subsequent to placing the first LED die on thesubstrate, repositioning the first LED die to align the electrodes withthe contacts, the flux or underfill remaining between the electrodes andthe contacts.